RE: 26 GT Problem
Nate,
I suppose your engine is an Evolution 26GT.
First, regarding the O-ring; newer engines use it to replace (not to augment...) the back-plate gasket.
So, the fact that your engine does have the O-ring means it neither needs, nor is it designed to use a gasket.
Also, the thickness of a compressed gasket is less than 0.1 mm and it is hardly the type of material to act as 'a spacer'...
It is much more probable that the rear bearing is prevented from being fully seated in the crankcase, by a piece of grit, or a burr.
...Or that the crankshaft is being prevented from being fully seated in the rear bearing's inner race by the same...
Also, there is an unlikely event of the big-end bearing bulging out and its retainer contacting the back-plate...
It would be very helpful, if you could take some good quality close-up photos, of the back-plate and of the open rear of the engine and post them in the thread, for our reference.